Light weight parallel manipulators using active/passive cables

ABSTRACT

The present invention provides parallel, cable based robotic manipulators, for use in different applications such as ultra high-speed robots or positioning devices with between three to six degrees of freedom. The manipulators provide more options for the number of degrees of freedom and also more simplicity compared to the current cable-based robots. The general structure of these manipulators includes a base platform, a moving platform or end effector, an extensible or telescoping central post connecting the base to moving platform to apply a pushing force to the platforms. The central post can apply the force by an actuator (active), or spring or air pressure (passive) using telescoping cylinders. The robotic manipulators use a combination of active and passive tensile (cable) members, and collapsible and rigid links to maximize the benefits of both pure cable and conventional parallel mechanisms. Different embodiments of the robotic manipulators use either active cables only, passive cables only, or combinations of active and passive cables. An active cable is one whose length is varied by means of a winch. A passive cable is one whose length is constant and which is used to provide a mechanical constraint. These mechanisms reduce the moving inertia significantly to enhance the operational speed of the robots. They also provide a simpler, more cost effective way to manufacture parallel mechanisms for use in robotic applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application relates to, and claims the priority benefitfrom, U.S. Provisional Patent Application Ser. No. 60/394,272 filed onJul. 9, 2002 and which is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to robotic manipulators for moving andpositioning an object in space, and more particularly the presentinvention relates to light weight cable actuated active/passive parallelmanipulators.

BACKGROUND OF THE INVENTION

Robotic manipulators may be divided into two main categories, paralleland serial manipulators. Serial manipulators, which are more common inthe industry, have several links in series usually connected by rotaryor sliding joints. They are analogous to the human arm which has aseries of links hinged at the shoulder, elbow and wrist. Theconfiguration of serial manipulators necessitates the location of thedriving motors to be at the joints themselves or the use of a heavy orcomplicated linkage for transferring the motion from the base of therobot to the joints. This is a disadvantage since it requires themovement of the large mass of the manipulator and drives even for asmall payload. Further, the positional error of the end effector of aserial manipulator is the accumulation of the errors in the individuallinks so that by increasing the size or number of links the errorassociated with the position of the end effector increases.

In contrast to serial manipulators, the links of a parallel manipulatorfunction in parallel to determine the movement of the end effector. Aflight simulator and camera tripod are two examples of this kind ofmechanisms. If one of the legs of a tripod is extended or moved, itchanges the position of the end point. Parallel manipulators haverelatively lower mass to payload ratio since the links work together andthe actuators are mounted on a stationary base. They also have betterprecision since the error in the end effector is in the same order ofactuators' error.

Low inertia, and therefore, high speed manipulation is one of the mainapplications of parallel robots. U.S. Pat. No. 4,976,5821 issued toClavel, entitled ‘Device for the Movement of and Positioning of anElement in Space’, and reported further in Clavel, ‘Delta, a Fast Robotwith Parallel Geometry’, Proceeding of International Symposium onIndustrial Robots, pp. 91-100, April 1988, discloses one of the mostsuccessful mechanisms of this kind which produces movement with threepure translational degrees of freedom at its end effector. In thismanipulator of Clavel, rotating arms are connected to the end effectorusing three parallelograms. The parallelograms constrain the endeffector to be parallel to the base plate at all times and therefore,three pure translational movements are achieved.

Other manipulator designs such as disclosed in L-W. Tsai, ‘Kinematic ofa Three-DOF Platform With Extensible Limbs’, Proceeding of theConference of Recent Advances in Robot Kinematics, pp. 401-410, 1996,also provide pure translational movement of the end effector with threetranslational degrees of freedom. In the Tsai mechanism, three linearactuators connect the end effector to the stationary platform withuniversal joints. The specific configuration of the universal jointsguarantees the three translational motions of the end effector.

There are also parallel mechanism robots with 6-DOF such as the hexapod, see Griffis M., Crane C., et Duffy J., ‘A smart kinestaticinteractive platform’, In ARK, pp. 459-464, Ljubljana, 4-6 Jul. 1994,and the hexa robot disclosed in U.S. Pat. No. 5,333,514 issued to Toyamaet al. entitled ‘Parallel Robot’.

In general, parallel mechanism robots have higher stiffness to weightratio, moment and torque capacity, and better accuracy. They alsobenefit from a simpler mechanism due to the elimination of drive trainsand, also lower moving mass due to the stationary location of theactuators. Further reduction in the moving inertia of parallelmechanisms may be achieved by replacing the rigid links with tensilemeans such as cables. Replacing the rigid arms not only reduces themoving inertia but it lowers manufacturing cost and simplifies themechanism structure by eliminating many joints.

Using cables in cranes such as disclosed in U.S. Pat. No. 3,286,851issued to J. R. Sperg entitled ‘Cargo Handling Rig’, and similarapplications, see U.S. Pat. No. 5,967,72910 issued to G. F. Foesentitled ‘Bottom Discharge Rotating Ring Drive Silo Unloader’, is olderthan robotics, however in recent years several attempts have been madeto design cable actuated manipulators. Some of these manipulators aredesigned to imitate human arms and can be considered as serialmanipulators with parallel actuators, see U.S. Pat. No. 3,631,737 issuedto F. E. Wells entitled ‘Remote Control Manipulator for Zero GravityEnvironment’; U.S. Pat. No. 3,497,083 issued to V. C. Anderson, R. C.Horn entitled ‘Tensor Arm Manipulator’; and U.S. Pat. No. 4,683,773issued to G. Diamond entitled ‘Robotic Device’.

A pure parallel cable actuated mechanism is disclosed in S. Kawamura, W.Choe, S. Tanaka, S. R. Pandian, ‘Development of an ultrahigh Speed RobotFALCON using Wire Drive System’, Proceeding of IEEE Conference onRobotics and Automation, pp. 215-220, 1995. This manipulator has sevenactive cables to provide 6-DOF for the end effector. This mechanism doesnot have any rigid link in its structure and the cables are extended inboth sides to maintain tension in the cables.

U.S. Pat. No. 4,666,362 issued to S. E. Landsberger and T. B. Sheridanentitled ‘Parallel Link Manipulator’ discloses a manipulator which usessix active cables and a passive collapsible link. The collapsible linkapplies a pushing force between the moving and stationary platforms inorder to keep all cables in tension.

U.S. Pat. No. 5,313,854 issued to H. A. Akeel entitled ‘Light WeightRobot Mechanism’, discloses another combined cable-collapsible mechanismwhich moves the end point of the collapsible shaft in the space but doesnot have any control on its orientation.

SUMMARY OF THE INVENTION

Based on the advantages of parallel and cable based manipulators, somenew designs are introduced in this work which can be used in ultrahigh-speed robots with 3 to 6 degrees of freedom. The robotic mechanismsdisclosed herein provide more options for the number of degrees offreedom and also more simplicity compared to the current cable-basedrobots. In the proposed designs a combination of active and passivetensile members, collapsible and rigid links are used to maximize thebenefits of both pure cable and parallel mechanisms.

Applications of both passive and active cables in the new designsimprove performance, simplicity and feasibility of the robots. An activecable is one whose length is varied by means of a rotating drum. Apassive cable is one whose length is constant and which is used toprovide a mechanical constraint. In general, compared to rigid linkparallel mechanisms the robotic mechanisms disclosed hereinadvantageously reduce the moving inertia significantly to enhance theoperational speed of the robots. They also provide a simpler, more costeffective way to manufacture parallel mechanisms for use in roboticapplications, measurements, and entertainments.

The design of new light weight parallel manipulators for high-speedrobots using active/passive cables is explained herebelow. The generalstructure of these manipulators has the following main components (seeFIGS. 1 and 2):

-   a) A base platform 24.-   b) A moving platform or end effector 22.-   c) An extensible or telescoping central post 26 connecting the base    24 to moving platform 22 to apply a pushing force to the platforms.    The central post can apply the force by an actuator (active) or    spring or air pressure (passive); and-   d) Active cables 28. Active cables are those whose lengths change    using an actuator; and/or-   e) Passive cables 42. Passive cables are cables whose lengths are    fixed.

The robotic mechanism may have just active cables, just passive cables,or a combination of both.

In one aspect of the invention there is provided a robotic mechanism,comprising:

a support base, an end effector and a biasing member having opposed endsand attached at one of said opposed ends to the support base andattached at the other of said opposed ends to the end effector; and

at least three cables each connected at a first end thereof to said endeffector and said at least three cables having second ends beingattached to an associated positioning mechanism for retracting ordeploying each of said at least three cables to position said endeffector in a selected position in space, said biasing member applyingforce on the end effector with respect to the support base formaintaining tension in said at least three cables.

The present invention also provides a robotic mechanism, comprising:

a support base, an end effector and a biasing member having opposed endsand pivotally attached at one of said opposed ends to the support baseand pivotally attached at the other of said opposed ends to the endeffector; and

six cables each connected at a first end thereof to said end effectorand said six cables having second ends being attached to an associatedpositioning mechanism for moving the second ends of the associated cableindependently of the other cables, said biasing member applying force onthe end effector with respect to the support base for maintainingtension in said six cables, wherein movement of the second ends of thecables by the associated positioning mechanisms changes a position andorientation of the end effector so that the robotic mechanism has sixdegrees of freedom.

The present invention also provides a five-degree-of-freedom roboticmechanism, comprising:

a support base, an end-effector and a biasing member having opposed endsand pivotally attached at one of said opposed ends to the support basewith a universal joint and pivotally attached at the other of saidopposed ends to the end-effector with a universal joint; and

five cables each connected at a first end thereof to said end effectorand said five cables having second ends being attached to an associatedpositioning mechanism for moving the second ends of the associated cableindependently of the other cables, said biasing member applying force onthe end effector with respect to the support base for maintainingtension in said five cables, wherein movement of the second ends of thecables by the associated positioning mechanisms changes a position andorientation of the end-effector

The present invention also provides a robotic mechanism, comprising:

an end effector, a post having opposed ends being pivotally connected atone of said opposed ends to the end effector;

a support base defining a plane and having a hole extendingtherethrough, an outer ring structure pivotally connected to saidsupport base within said hole for pivotal motion of said outer ringstructure out of the plane of said support base, a first actuator forpivoting said outer ring structure, an inner ring structure pivotallymounted to said outer ring structure inside said outer ring structure,said inner ring structure being concentric with said outer ringstructure, a second actuator for pivoting said inner ring structure,said inner ring structure having an axis of rotation in the plane of theouter ring, and perpendicular to the axis of rotation of said outer ringstructure, said inner ring structure having a central web with a holetherethrough and a universal joint mounted in said hole to the centralweb, the other end of said post being slidably mounted in said universaljoint, bias means connected to said post for biasing said end effectoraway from said support base;

a first set of three cables each connected at one end thereof to saidend effector and the other ends of said first set of three cables beingattached to positioning means mounted on said support base for pullingsaid three cables independently of each other to position said endeffector in a selected position in space; and

a second set of three cables each connected at one end thereof to saidend effector and the other ends thereof being attached to the other endof said post, said second set of three cables being mounted to saidinner ring at substantially 120° with respect to each other andconstrained to be parallel to each other between said end effector andsaid inner ring and wherein when said positioning means moves said endeffector to a selected position in its workspace, said second set ofthree cables maintains said end effector in a plane parallel to theplane of said inner ring.

The present invention also provides a robotic mechanism, comprising:

an end effector, a post having opposed ends being pivotally connected atone of said opposed ends to the end effector using a universal joint,the post having an adjustable length;

a support base defining a plane and having a hole extendingtherethrough, an outer ring structure pivotally connected to saidsupport base within said hole for pivotal motion of said outer ringstructure out of the plane of said support base, a first actuator forpivoting said outer ring structure, an inner ring structure pivotallymounted to said outer ring structure inside said outer ring structure,said inner ring structure being concentric with said outer ringstructure, a second actuator for pivoting said inner ring structure,said inner ring structure having an axis of rotation in the plane of theouter ring, and perpendicular to the axis of rotation of said outer ringstructure, said inner ring structure having a central web with a holetherethrough and a universal joint mounted in said hole to the centralweb, the other end of said post being slidably mounted in said universaljoint;

a first set of three cables each connected at one end thereof to saidend effector and the other ends of said first set of three cables beingattached to a positioning mechanism mounted on said support base forpulling said three cables independently of each other to position saidend effector in a selected position in space; and

a second set of three cables each connected at one end thereof to saidend effector and the other ends thereof being attached to, a winchmounted on said central web of the inner ring assembly, said second setof three cables being guided through pulleys mounted to said inner ringat substantially 120° with respect to each other and constrained to beparallel to each other between said end effector and said inner ring,wherein the winch retracts or deploys all three cables simultaneouslyand keeps the cable lengths between the inner ring and the end-effectorequal so that when said positioning mechanism moves said end effector toa selected position in its workspace, said second set of three cablesmaintains said end effector in a plane parallel to the plane of saidinner ring.

The present invention also provides a robotic mechanism, comprising:

an end effector, a post having opposed ends and an adjustable lengthbeing pivotally connected at one of said opposed ends to the endeffector;

a support base, the other end of said opposed ends of the post beingpivotally connected on a top surface of said support base;

a set of three cables each connected at one end thereof to the end ofsaid post pivotally connected to said end effector and the other ends ofeach of said first set of three cables being attached to positioningmeans mounted on said support base for pulling said cables to positionsaid end effector in a selected position in space;

a first longitudinal shaft having a first longitudinal axis and a pulleybeing rigidly mounted on each end of said first shaft, said firstlongitudinal shaft being mounted on a bottom surface of said supportbase and parallel to said support base, the first longitudinal shaft ispassing through a first sleeve, a first rotational spring mounted fromone end to the first sleeve and from the other end to the firstlongitudinal shaft for applying a constant torque to the firstlongitudinal shaft, including a first motor connected to said firstlongitudinal shaft for rotating said first longitudinal shaft about anaxis parallel to the said support base and normal to said firstlongitudinal shaft, a second longitudinal shaft having a secondlongitudinal axis and a pulley rigidly mounted on each end of saidsecond shaft, said second longitudinal shaft being mounted on the bottomsurface of said support base and parallel thereto and oriented so saidfirst longitudinal axis is perpendicular to said second longitudinalaxis, the second longitudinal shaft is passing through a second sleeve,a second rotational spring mounted from one end to the sleeve and fromthe other end to the second longitudinal shaft applies a constant torqueto the second longitudinal shaft, including a second motor connected tosaid second longitudinal shaft for rotating said second longitudinalshaft about an axis parallel to the said support base and normal to saidsecond longitudinal shaft; and

a first pair of cables with each cable connected at one end thereof tosaid end effector and the other end of one of the cables being collectedby one of the pulleys at the end of the first longitudinal shaft and theother end of the other cable being collected by the other pulley at theother end of the first longitudinal shaft, the first rotational springmounted in the first sleeve 148 which applies torque to the firstlongitudinal shaft has both the pulleys rotate and collect the firstpair of cables so that the lengths of the cables of the said first pairof cables remain the same and therefore a parallelogram is maintained bythe first pair of cables, a second pair of cables with each cableconnected at one end thereof to said end effector and the other end ofone of the cables being collected by one of the pulleys at the end ofthe second longitudinal shaft and the other end of the other cable beingcollected or deployed by the other pulley at the other end of the secondlongitudinal shaft as said second longitudinal shaft is rotated by thetorque provided by the rotational spring mounted in the second sleeve146 and therefore the length of the cables of said second pair of cablesremains the same and thus a parallelogram is maintained by the secondpair of cables, and wherein said cables of said first pair of cables areparallel and said cables of the second pair of cables are parallel sothat a plane defined by said end effector is maintained parallel to aplane defined by said two longitudinal shafts.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only,reference being had to the accompanying drawings in which:

FIG. 1 is a perspective view of a three degree of freedom (DOF) wireactuated parallel robot using active cables constructed in accordancewith the present invention;

FIG. 2 is a perspective view of a three degree of freedom wire actuatedparallel robot using passive cables;

FIG. 3 is a perspective view of another embodiment of three degree offreedom wire actuated parallel robot using passive cables;

FIG. 4 is a perspective view of a six DOF parallel mechanism usingpassive cables;

FIG. 5 is a perspective view of a three-to-five DOF parallel mechanismusing active and passive cables;

FIG. 6 shows a top view (view A-A in FIG. 5) of the base platform andrings of the mechanism of FIG. 5;

FIG. 7( a) shows an overall perspective view of the configuration ofactive cables in the mechanism of FIG. 5;

FIG. 7( b) shows a detailed view of the portion of FIG. 7( a) in thesquare box;

FIG. 8( a) shows an overall perspective view of the configuration of thepassive cables in the mechanism of FIG. 5;

FIG. 8( b) shows a detailed view of a portion of the passive cablemechanism of FIG. 8( a);

FIG. 8( c) shows a side view of the passive cable configuration of FIG.8( a);

FIG. 9 is a perspective view showing the connection of passive cables tothe bottom end of the center post;

FIG. 10 shows the mechanism of FIG. 5 in two positions, vertical andtilted at an angle from the vertical showing the moving platform remainsparallel to the base platform;

FIG. 11( a) is an overall perspective view of a three-to-five DOFrobotic mechanism;

FIG. 11( b) is a close up detailed perspective view of the wiretensioning mechanism of the robotic mechanism of FIG. 11( a);

FIG. 12 is a top perspective view of the mechanism of FIG. 11 a absentthe end effector and central post showing the tensioning mechanism forthe passive cables used to maintain the moving platform parallel to thebase;

FIG. 13 shows the configuration of active cables for positioning thecentral post of the mechanism of FIG. 11;

FIG. 14 is a perspective view of a hybrid parallel mechanism using sevenactive cables that can produce between three and five degrees of freedomfor the moving platform;

FIG. 15 is a perspective view of the central extensible rod and threeactive cables for the mechanism of FIG. 14;

FIG. 16 is a bottom view of the mechanism of FIG. 14;

FIG. 17 is a perspective view of three degree of freedom parallel planarmanipulator using active cable;

FIG. 18 is a bottom view of the moving platform component connection forplanar manipulator;

FIG. 19 is a perspective view of two degree of freedom parallel planarmanipulator using an active cable;

FIG. 20 is a perspective view of a parallel planar manipulator using apassive cable;

FIG. 21 is a bottom view of three degree of freedom parallel planarmanipulator using a passive cable;

FIG. 22 shows the parallelism of the moving platform enforced by twoparallelograms;

FIG. 23 is a perspective view of a two degree of freedom parallel planarmanipulator driven by passive cables with the orientation constrained bya winch mechanism; and

FIG. 24 shows a perspective view of a three degree of freedom parallelplanar manipulator driven by passive cables with the orientationcontrolled by a cam and a winch mechanism.

DETAILED DESCRIPTION OF THE INVENTION

1. Three-Degree-of-Freedom Parallel Mechanism Using Active Cables

A three-degree-of-freedom parallel robotic mechanism using active cablesconstructed in accordance with the present invention is shown generallyat 20 in FIG. 1 and includes a moving platform 22 that is attached tobase platform 24 using an extensible or telescoping central post 26 andthree sets of parallel cables 28 with one end of each of cable attachedto platform 22 and the other ends of each pair of cables attached to anassociated winch assembly 30. Each winch assembly 30 includes a drum 32mounted for rotation in a frame 36 which is attached to the base 24 tokeep the drum 32 in place and also to guide the cables 28 to the drumvia two holes 39 located in the top plate 38 of the frame 36. Theextensible post 26 is attached to the platform 22 (end-effector) andbase 24 by universal joints 34 at both ends of the post to prevent therotation of the moving platform 22. The extensible center post 26applies a compression force between the platforms 22 and 24 using alinear actuator such as a hydraulically, pneumatically, and electricallypowered cylinder. Alternatively, a linear motor (active element) orusing a preloaded spring, or air pressure (passive element) may beemployed in alternative embodiments of the mechanism to maintain tensionof cables 28. Post 26 may be any one of a hydraulically, pneumatically,and electrically powered cylinder.

The motion of the moving platform 22 is controlled by the three pairs ofactive cables 28. The two cables of each pair of cables 28 are parallelto each other to make a parallelogram as shown by the closed loop ofa-b-c-d in FIG. 1. A motor controller 31 is connected to the motors 33for driving the motors as well as being connected to position/velocitysensors on each of the drums 32. A computer 35 attached to thecontroller 31 is used to program/command the controller for positioningthe cables on each of the winches. A tool 37 is mounted on top of endeffector 22 and is controlled by controller 31 or by separate controller41. When the end effector 22 is to be positioned in a selected locationin its workspace, signals are sent by controller 31 based on itsexisting program or command signals sent by computer 35 which in turnmoves the drums 32 in each winch 30 to either roll up the parallelcables 28 or release them, depending on the particular winch and wherein the robotic workspace space the end effector 22 is to be located. Thelengths of the three pairs of the cables 28 are adjusted independentlyto provide three degrees of freedom to the end effector platform 22.

Due to the three cable-parallelogram structures the moving platform 22will always be parallel to the base platform 24 and can undergo threetranslational degrees of motion. This is obtained because the edge a-bin parallelogram a-b-c-d (similarly in the other two parallelograms) isalways parallel to edge c-d that is parallel to base platform 24. Sincethe three intersecting edges (a-b and the other two similar edges) arealways parallel to base platform 24, the moving platform 22 remainsparallel to base platform 24 regardless of the lengths of the pairs ofcables 28. The lengths of each pair of cables 28 are controlledindependently by their associated rotating drums 32. The lengths of eachpair of cables 28 determines the center location of the moving platform22 while the parallelograms keep the platform 22 parallel to the base24. The length of the central post 26 changes according to the locationof the moving platform 22 and the compression force that is applied tothe platform 22 from the central post 26.

2. Three-Degree-of-Freedom Parallel Mechanism Using Passive Cables

A three-degree-of-freedom parallel robotic mechanism using passivecables constructed in accordance with the present invention is showngenerally at 40 in FIG. 2 and includes moving platform 22 that isattached to base platform 24 using an extensible or telescoping centralpost 26. As with robot 20 in FIG. 1, the extensible post 26 is attachedto the platforms 22 and 24 by universal joints 34 at both ends of thepost to prevent the rotation of the moving platform 22. There are threepairs of fixed-length cables 42 attached to the moving platform 22 andeach pair of cables 42 forms a parallelogram a-b-c-d as seen in FIG. 2.The ends of each pair of cables 42 at the lower edge c-d of theparallelogram are connected to a link arm 44 using a revolute joint 46having an axis of rotation coincident with c-d. Each link arm 44 isconnected to a bracket 48 using another revolute joint 50 whose axis ofrotation is parallel to axis c-d. Frame 48 is attached to base 24 andlink arm 44 is rotated by an actuator such as an electrical motor (notshown in the figure). When link arm 44 is rotated about the rotationalaxis of the lower revolute joint 50, the upper axis a-b remains parallelto axis c-d which guarantees the moving platform 22 stays parallel tothe base platform 24 during any motion.

The same reasoning as to why the moving platform 22 remains parallelwith the base 24 in apparatus 20 in FIG. 1 applies to base 24 andplatform 22 of apparatus 40 regardless of the angles of arms 44. Thusplatform 22 has a pure translational motion along the X, Y and Z-axes.The extendable center post 26 pushes the platform 22 away from the base24 and generates tension in the pairs of cables 42 which prevents themfrom becoming slack.

FIG. 3 shows an alternative embodiment at 60 of a robot constructedfollowing the same principle as robot 40 with the difference being linkarm 44 (FIG. 2) is replaced by actuators that move edge c-d and theother two similar axes of the parallelograms parallel to the baseplatform. As an example, connection rod 46 can be moved horizontally orvertically by a linear actuator attached thereto (not shown) to changethe location of rod 46 without modifying its angle with the base 24.Similarly, connection rod 46 can be attached to a rotary actuator formovement in a plane parallel to the base platform 24 to provide thedesired movement of the platform 22. For all these different motions aslong as the axis of connection rods 46 are maintained parallel to thebase platform 24 the mechanism 60 will have three translational degreesof freedom in the X, Y and Z directions.

Mechanisms 40 and 60 also include a computer controlled motor controller(not shown) such as computer 35 connected to controller 31 shown in FIG.1.

3. Six-Degree-of-Freedom Parallel Mechanism Using Passive Cables

A generalization of the design shown in FIG. 3 can be extended to a 6degree of freedom robot as shown generally at 66 in FIG. 4. In thisdesign the extendible center post 26 is attached to the base 24 andmoving platform 22 by two spherical joints 56, or one spherical jointand one universal joint instead of two universal joints as is used inmechanisms 20, 40, and 60 in FIGS. 1, 2, and 3. The parallelograms inthe previous mechanisms 20, 40 and 60 defined by the pairs of parallelcables are used to impose mechanical constraints to eliminate threerotational degrees of freedom. In the six degree of freedom robot 66 theends of cables 42 are connected to separate actuators to provide threeextra degrees of freedom. In this design the six cables 42 are stillpassive and are connected at one end to an associated arm 44 and at theother end to moving platform 22. Each link arm 44 is connected to aframe 48 with a revolute joint 50. Frame 48 is attached to the base 24and link arm 44 is rotated by an actuator such as an electrical motornot shown but similar to the motors and controller shown in FIG. 1. Whenlink arm 44 is rotated the end points of the cables connected to arms 44change and as a result the position and orientation of the movingplatform 22 can be controlled. The central extensible post 26 applies apushing force through a spring or air cylinder (not shown in the figure)to keep cables 42 in tension. It should be noted that the design is notlimited to the use of assembly 44, 48 and 50 to move the end points ofthe cables and any mechanism and actuator (linear or rotary) can be usedto achieve the same number of degrees of freedom, as discussed withrespect to the mechanism of FIG. 3. Also, there are no limitations onthe location of cable 42 attachment to the moving platform, however,these locations will change the overall workspace of the robot.Mechanism 66 also include a computer controlled motor controller (notshown) such as computer 35 connected to controller 31 shown in FIG. 1for controlling each of the actuators.

The six degree-of-freedom robotic mechanism of FIG. 4 may be convertedto a five degree-of-freedom device by replacing spherical joints 56connecting post 26 to base 24 and end effector 22 with universal jointsand removing one of the six cables 42 and associated link arm 44 andmotor. The five degrees of freedom will include three translational andtwo rotational motions (pitch and yaw). The replacement of the sphericaljoints with universal joints will eliminate the roll motion of moving,platform 22 with respect to post 26 and fixed platform 24.

4. Three-to-Five DOF Parallel Mechanism Using Passive and Active Cables

Referring to FIG. 5, there is shown generally at 70 a hybrid parallelmechanism using a combination of active and passive cables to providefive degrees of freedom for moving platform 22, including threetranslational and two rotational motions. In this embodiment of theinvention, base platform 24 includes two rings 76 and 74. The top viewof base 24 and the two rings is shown in FIG. 6. Ring 76 is attached tobase platform 24 by two revolute joints 87 diametrically located onopposite sides of ring 76 and having coextensive or coincident axis ofrotation. Revolute joints 87 are fixed in ring 76, and held by collarson base 24.

Actuator 84 is mounted on base 24 and its shaft is connected to one ofthe revolute joints 87 to provide a relative rotational motion of ring76 with respect to base 24 so that ring 76 can be rotated out of theplane of base 24. Similarly, ring 74 is attached to ring 76 by tworevolute joints 86 diametrically located on opposite sides of ring 74and with revolute joints 86 having coextensive or coincident axis ofrotation. The revolute joints 86 are fixed in ring 76 and held bycollars in ring 74. The coextensive axes of rotation of the two revolutejoints 86 are normal to the coextensive axes of rotation of the tworevolute joints 87. Actuator 82 is mounted on ring 74 and its shaft isconnected to one of the revolute joints 86 to provide a relativerotational motion between rings 74 and 76 for rotating ring 74 out ofthe plane defined by ring 76. As a result, ring 74 is connected to base24 through ring 76 and has two rotational degrees of freedom (pitch andyaw) and its orientation is set by motors 82 and 84.

At the center of ring 74 there is collar 78 which is attached to ring 74by a universal joint 80. When the planes of rings 74, 76 are in the sameplane as base 24 and collar 78 is normal to the base the axes ofrotation of universal joint 80 and revolute joints 86 and 87 are all ina single plane. Also, center post 72 can only slide in collar 78 withoutany rotation. Platform 22 (FIG. 5) is connected to center post 72 byuniversal joint 89 (FIG. 7( a)). Universal joint 89 prevents therotation of platform 22 with respect to the longitudinal axis of centerpost 72.

Referring again to FIG. 7( a), the top end of center post 72 is attachedto three active cables 88 which are used to orient the center post 72 inspace. FIG. 7( a) shows the mechanism without the passive cables 98 andmovable platform 22 to show more clearly the active cables 88. Theactive cables 88 are attached at one end thereof to the tip of centerpost 72. Referring particularly to FIG. 7( b), each of the active cables88 is pulled and accumulated using an associated winch assembly thatincludes a pulley 92 and a motor 90 which rotates the pulley. Pulley 92and motor 90 of each winch assembly is mounted in housing 96 which isattached to the base platform 22 and each of the cables 88 passesthrough a hole 94 located in the top of the associated housing 96. Thetip of center post 72 can be moved to any point in the workspace bychanging the length of active cables 88. The center post 72 applies apushing force to cables 88 to keep them in tension at all times. Thisforce can be generated by means of passive elements such as spring 73which applies the force between collar 78 and center post 72. In analternative embodiment an active element such as a linear motor (notshown in the figures) may be used instead.

There are three passive cables 98 (best seen in FIGS. 8( a) and 8(b))attached at one end to the moving platform 22 and at the other end tothe bottom end of center post 72 (see FIG. 9). Passive cables 98 areparallel to each other in the section between ring 74 and platform 22(FIG. 10) and are used to maintain the moving platform 22 parallel toring 74 so that any orientation of ring 74 transfers to platform 22.

Referring to FIG. 8( a), the passive cables 98 from platform 22 areguided through pulleys 100 which are mounted to brackets 103 (see FIG.8( b)), which in turn are attached to ring 74 using revolute joints (notshown). The revolute joints allow the pulleys 100 to adjust themselveswith respect to the direction of the associated cables 98.

Three other pulleys 104 (see FIG. 9) are mounted in brackets 106 whichare mounted on a frame 108 which is attached to collar 78. The axes ofpulleys 100 are in the same plane which passes through the center ofuniversal joint 80 (FIG. 6). Also, the axes of pulleys 104 are in thesame plane which passes through universal joint 80. These conditions arerequired to keep the platform 22 parallel to ring 74.

Pulleys 104 guide the cables 98 to their attachment point at the bottomend of center post 110. Three springs 112 are in series with cables 98.These three springs 112 are used to provide tension in passive cables 98and also compensate for small changes in the length of cables 98 whenthe center post 72 deviates from its vertical position.

The three passive cables 98 maintain the platform parallel to ring 74 asshown in FIG. 10 for a 2D situation. In an ideal configuration, pulleys100 and 104 have zero diameters. As seen in the figure, regardless ofthe angle of 72 BC=EF and DC=DE. Since the overall length of the cablesABCD and GFED are equal, AB=GF all the time. This constitutes aparallelogram which guarantees end effector 22 stays parallel to baseplatform 24.

The embodiment shown at 70 in FIG. 5 is a five degree-of-freedommechanism that has three translational motions of the moving platform 22that are provided by actuators 90 and active cables 88, and the tworotational degrees of freedom are provided by actuators 82 and 84 toorient moving platform 22. The translational and rotational motions ofthe moving platform are independent which result in simple kinematics ofthe mechanism. Mechanism 70 can be converted into a three degree offreedom mechanism by removing rings 74 and 76 and connecting pulleys 100and their frames directly to base 24. In this configuration platform 22is always parallel to the base and its location can be changed by activecables 88 and motors 90. Alternatively, a three degree of freedommechanism can be obtained by locking rings 74 and 76 with respect tobase 24.

5. Alternative Three-to-Five DOF Parallel Mechanism Using Active Cables

Referring to FIG. 11( a), there is shown generally at 200 a hybridparallel mechanism using a combination of active and passive cables toprovide five degrees of freedom for moving platform 22, including threetranslational degrees of freedom and two rotational degrees of freedom.The overall structure of mechanism 200 is very similar to mechanism 70in FIG. 5 except for the central post 26 and the way passive cables 98keep the moving platform 22 parallel to ring 74. The central post inthis design is extensible and connected to both moving platform 22 andring 74 with universal joints. It further applies an active or passivepushing force to the platform and ring via a spring or air cylinder (notshown in the figure) or it could be a linear motor to continuouslyadjust the force.

A close-up of the mechanism that keeps platform 22 parallel to ring 74is shown in FIGS. 11 b and 12. Passive cables 98 are guided to a winchmechanism which includes a drum 97 mounted for rotation in a frame 107and driven by a motor 99. Frame 107 is attached to ring 74. Threepulleys 100 are mounted on frames 106 that are connected to ring 74 byrevolute joints 103 and spaced 120° with respect to each other aroundring 74. Two pulleys 101 are mounted on associated frames 105 that areconnected directly to ring 74. These two pulleys 101 receive two of thecables 98 from two of the pulleys 100 which are then wrapped on drum 97.Cable 98 from the third pulley 100 goes directly to drum 97, best seemin FIG. 12. The cables 98 are wound on drum 97 by applying a torquegenerated by passive elements like rotational springs or active elementssuch as electrical or air motors shown schematically by 99. As seen inFIG. 12 the lengths of cables 98 between pulleys 100 and drum 97 areindependent from the position and orientation of platform 22. Also,cables 98 are wrapped around one single drum 97 and as a result thechange in the lengths of cables 98 between pulleys 100 and platform 22will be the same in any robot's configurations. Now, if cables 98 areattached to platform 22 such that their lengths between pulleys 100 andconnection points on platform 22 become equal and parallel to thecentral post 26, each two cables 98 will make a parallelogram andtherefore platform 22 will remain parallel to ring 74 regardless of itsposition in the workspace.

FIG. 13 shows the arrangement of the active cables 88 that are the sameas the arrangement of the active cables in mechanism 70 in FIG. 7( a).Referring again to FIG. 11 a, mechanism 200 is a five degree of freedommechanism that includes three translational degrees of freedom of themoving platform 22 provided by actuators 90 and active cables 88, andthe two rotational degrees of freedom provided by actuators 82 and 84 toorient moving platform 22 in its workspace. The translational androtational motions of the moving platform 22 are independent of eachother which results in simple kinematics of the mechanism. Mechanism 200may be converted into a three degree of freedom mechanism by removingrings 74 and 76 and connecting pulleys 100 and their frames directly tobase 24. This way platform 22 is always parallel to the base 24 and itslocation can be changed by changing the length of active cables 88 usingmotors 90.

In summary, the embodiment shown in FIGS. 11, 12 and 13 is a 5 dofmechanism. In this mechanism the second set of cables are not attachedto the bottom end of the post. They are pulled and collected by winch97. There are five pulleys mounted on the inner ring in order to guidethe three cables to the winch. This winch pulls and collects all threecables simultanously and hence keeps the cable lengths between the innerring and the end-effector equal. Therefore, the end-effector staysparallel to the inner ring plane. Winch 97 can be connected to a motoror to a rotational spring in order to pull cables and keep them intension. In this mechanism the post can be as simple as the mechanismsof FIGS. 1 to 5.

6. Three-to-Five DOF Parallel Mechanism Using Active Cables

FIG. 14 shows a hybrid parallel mechanism at 120 using seven activecables that can produce between 3 and 5 degrees of freedom for themoving platform 22. In this embodiment, the moving platform 22, baseplatform 24, and extensible center post 26 and universal joint 34 aresimilar to the previous embodiments. Three active cables 122 as shown inFIGS. 14 and 15 are attached at one end to the top of extensible centerpost 26 and the other ends are attached to winches 124 which are mountedin bracket frames 126 attached to platform 24. Winches 124, whichcontrol the lengths of cables 122 control the end location of theextensible rod in the space.

Referring particularly to FIGS. 14 and 16, two pairs of cables 130 and132 form two parallelograms. The pair of cables 130 are pulled andcollected by two pulleys 136 mounted on the ends of shaft 138. The pairof cables 132 are pulled and collected by two pulleys 140 mounted on theends of shaft 142. Both shafts 138 and 140 and the associated pulleysmounted on the ends of the respective shafts form a single body andtherefore, the two pulleys rotate simultaneously with the shaft. Shaft142 rotates inside collar 146. There is also a source of constant torqueacting between shaft 142 and collar 146. This torque can be applied by aspring which maintains the cables 132 in tension. Similarly, shaft 138rotates inside a collar 148. There is also a source of constant torqueacting between shaft 138 and collar 148 which may be applied by a springand this keeps the cables 130 in tension. Maintaining the shafts 138 and142 parallel to base 24 and platform 22′ ensures that the platform 22 isparallel to the base 24. Collars 146 and 148 are mounted to frame 150and collar 146 is connected to motor 152 and collar 148 is connected tomotor 154. The motors rotate the collars connected thereto and thisrotation is directly transferred to the platform 22 which alters theorientation of the platform 22.

Each of the two longitudinal shafts 138 and 142 mounted on the bottomsurface of the support plane are responsible for forming aparallelogram. Each of these two shafts has two pulleys rigidlyconnected at the two ends. The two shafts are initially parallel to thesupport base plane and normal to each other. In FIG. 16, there are twosleeves shown as 146 and 148. The two shafts pass through these sleevesand can rotate about their longitudinal axis. There are also rotationalsprings (not shown in the figure) used to apply a torque between eachsleeve and its associated shaft. Therefore, the shafts are under apassive torque so that they pull and collect the cables. As a result,the two pairs of parallel cables remain in tension and build twoparallelograms which force the end-effector to be parallel with the twolongitudinal shafts. If we rotate sleeves 146, 148 about an axisparallel to the support base plane and normal to the longitudinal axesof the shafts using motors 152 and 154, the rotation will be directlytransferred to the end-effector because the end-effector has to stayparallel to the longitudinal axes of the shafts. Therefore, the twomotors control the orientation of the end-effector and the mechanismwill provide 5 degrees of freedom.

7. Three DOF Planar Parallel Mechanism Using Active Cables

A general three degree of freedom planar parallel mechanism using activecables constructed in accordance with the presented invention is showngenerally at 170 in FIG. 17. The moving platform, 22 is attached to abase plate 172 by extensible or telescoping central post 174 and threeactive cables 176, through a winch assembly. See FIG. 18 for details.The base plate 172 provides a reference for the moving platform 22, andits function is identical to the base platform 24 of FIG. 1. The centralpost 174 is connected by revolute joint 180 to the bottom of movingplatform 22 having an axis of rotation 179 (see FIG. 18 for details),and base plate 172 by a revolute joint 178 with the pivoting axes 179 ofthe revolute joints 178 and 180 being perpendicular to the workspace ofthe robot. The out of plane moment induced on the moving platform 22 iscounter-balanced by these revolute joints. A clevis pin type of revolutejoint is a reasonable choice for this component. The cables 176 do notneed to be coplanar but they must be held in tension. Cables 176 may beattached to platform 22 by revolute joints 183 having axis of rotationparallel to axis 179 of joint 180. The purpose of the revolute joints183 is to reduce the amount of bending at the attachment points on thecables 176 to platform 22 which can increase the life span of the cablesand joints. Other attachment devices such as eyelets may be used as wellto reduce the bending while using the same design. The central post 174is used to exert a tensile force on the cables 176.

Each of the three winch assemblies 188 used in apparatus 170 comprises adrum 190 in a housing 192 with each drum being driven by a motor 194,with each housing 192 having a pilot hole 196 in its top surface throughwhich the associated cable 176 passes to be wound on drum 190. Thismechanism uses a pair of cables 176 (hence two winch assemblies 188) onone side of the central post 174 and at least one cable 176 and itsassociated winch 188 on the opposite side of post 174. As the motor 194turns, the drum 190 takes up or releases its associated cable 176. Thepilot hole 196 is used to position and set a reference point for thecables. The positioning of the moving platform 22 is controlled directlyby the amount of cable released by the drum. A computer controlled motorcontroller systems (not shown) such as computer 35 connected tocontroller 31 shown in FIG. 1 is used to adjust the length of the activecables.

In mechanism 170 shown in FIG. 17, the two parallel cables are similarto the parallelograms in the other embodiments and as long as theirlengths remain the same the end effector 22 can only move parallel tothe base. However, in this design we have considered two motors to beable to change both the orientation and location of the end effectorthrough three actuators.

8. Two DOF Planar Parallel Mechanism Using Active Cables.

In mechanism 170 of FIG. 17, the cables 176 from the side of post 174having the two winches 188 side-by-side have the ability to constraintthe orientation of the moving platform 22. If these cables are equal inlength, the cables 176 and the moving platform 22 forms a parallelogramfor the same reasoning as the apparatus shown in FIG. 1. Thus, themoving platform 22 will be parallel to the top plane 173 of the baseplate 172. On the other hand, if cables 176 are different in length, thecombination of all three cables determines the orientation of the movingplatform 22. Therefore, referring to FIG. 19, a two translational degreeof freedom active cable mechanism shown generally at 200 can beconstructed by replacing the two adjacent winch assemblies 188 shown onFIG. 17 with a two cable winch assembly 30 shown in FIG. 1. Note thatthe resulting mechanism requires only two motors 194 and 33 only. InFIG. 19, a design with one drum and motor for the two cables on the sameside of post 174 maintains the orientation of end effector 22 is fixed,which is parallel to the base 172 in FIG. 19. One of the two pairedcables could be longer or shorter with respect to the other therebyinclining the end effector 22 and as long as the length ratio of the twocables remains fixed the orientation or angle of the end effector 22will remain constant.

9. Three DOF Planar Parallel Mechanism Using Passive Cables

A general three degree of freedom planar parallel mechanism usingpassive cables in accordance with the present invention is showngenerally at 210 in FIG. 20. The moving platform 22 is attached to thebase plate 172 by extensible or telescoping central post 174 and threepassive cables 212 each connected at one end of the cables to threelink-arms 214 and the other ends connected to platform 22. Theconnections of the cables 212 and the central post 174 to movingplatform 22 is identical to the connections in mechanism 170 shown inFIGS. 17, 18 and 19. The connection of post 174 to base 172 is also thesame as in FIG. 17. Link-arms 214 are pivotally connected to base 172through revolute joints 218. Similar to the active cable counterpartmechanism 170 in FIG. 17, passive cable mechanism 210 also requires apair of the cables 212 on one side of the central post 174 and at leastone cable 212 on the opposite side. The side with two cables 212controls the orientation of the moving platform 22. If these cables wereequal in length and are parallel to each other, the cables and the tipsof the link-arms form two parallelograms. Therefore, the orientation ofmoving platform 22 will be fixed during movement of the end effector 22,and in the FIG. 20 it will be parallel to ground. On the other hand, ifthis pair of cables 212 is orientated differently, the combination ofall three cables determines the orientation of the moving platform 22.It should be pointed out that the motion of ends of the cables 212attached to arms 214 is not necessarily circular provided by arm 214,and it can be linear or any other complex trajectory generated bylinkage mechanisms. This is analogous to the motion of pins 46 in themechanism 60 illustrated in FIG. 3.

Referring to FIG. 21, a computer controlled motor controller system suchas computer 35 connected to controller 31 shown in FIG. 1 is used tocontrol the motor which drives the link arms 214. FIG. 21 shows a bottomview of the mechanism 210 with the motors 33 attached to the lowerrevolute joints 218 of the link-arms 214. The rigid link arms 214 areoffset to maximize the rotation of link arms 214 without anyinterference with each other. Increasing the rotation of link arms 214will minimize the size of the robot. This applies to the embodimentsshown in FIGS. 17 to 24. The orientation of the cables 212 is determinedby the amount of rotation on the link-arms 214. Coupled with the passivecables 212, the position and the orientation of the moving platform 22are controlled. The operating principal is similar to the mechanismillustrated in FIG. 2.

9. Two DOF Planar Parallel Mechanism Using Passive Cables

The mechanism shown in FIG. 20 can be converted to a two degree offreedom planar manipulator by synchronizing the motion of the pairedlink-arms. A timing belt (or equivalently a chain-sprocket drive) can beused for that purpose. The configuration can be made by attaching asheave to the revolute joint 218 and rigidly attach them to the link arm214. The synchronizing motion can be achieve by connecting the sheavewith a timing belt. A synchronized motion of the paired link-arms 214ensures the parallelism of the paired cables 212 that in turn restrictsorientation of the moving platform 22. As illustrated in FIG. 22, whentwo link-arms 214 are parallel, the close loops B-C-E-F and A-D-B-C formtwo parallelograms, which forces line A-D (attached to the movingplatform) to be parallel with line E-F (attached to the base plate).Hence, the rotating degree of freedom of the moving platform iseliminated, leaving two translational degrees of freedom to themechanism only.

10. Hybrid Two DOF Planar Parallel Mechanism Using Passive Cables forPositioning and Active Cable for Orientation

FIG. 23 shows another alternative embodiment of a mechanism shown at 220to achieve the parallelism of the moving platform 22. In mechanism 220,the cables 212 that are attached to the moving platform 22 are connectedto a beam 222, which pivots about the free end of a link-arm 224. Theorientation of the beam 222 is constrained using a winch assembly 226that includes a pair of cables 228 attached to beam 222, a drum 230, anda torsion spring (represented by a torsion load 232). Since both cables228 are connected to the same drum 230, their lengths are always equalto each other. The torsion spring 232 is attached to the drum 230 tomaintain tension in cables 228. Note that drum 230 is passive and itsrotation depends on the orientation of arm 224 orientation. Analogous tothe configuration shown in FIG. 22, the drum 230, the beam 222, thepairs of cables 228 and 212, and the moving platform 22 form twoparallelograms that ensure the parallelism between the moving platform22 and the base plate 172. Hence, the orientation of the moving platform22 is maintained parallel to the ground.

11. Hybrid Three DOF Planar Parallel Mechanism Using Passive Cables ForPositioning And Active Cable For Orientation

Referring now to FIG. 24, another embodiment of the mechanism shown inFIG. 20 is shown at 240. Mechanism 240 is similar to mechanism 220 ofFIG. 23 but includes a cam 242 that routes one of the cables 228. Theobjective of cam 242 is to create a bias on the length of one of theactive cables 228 to provide a new degree of freedom to the robotmechanism of FIG. 23. Adjusting the bias in the cable will allow tocontrol the orientation of the moving platform 22. The operatingprincipal is similar to a cam-follower mechanism. The linear guide 119is used to induce a linear motion to cam 242 as shown in FIG. 24.

When the cam 242 moves towards the center of the mechanism, it routesthe inner active cable 228 around the cam face. This effectivelyshortened the length the routed active cable while leaving the otheractive cable untouched. The resulting effect is a distortion on theparallelogram formed by the active cables and the beam. The routed cablepulls the beam on one side and forces the beam to tilt towards therouted cable. As a result, the beam 222 will no longer be parallel toground, but is controlled by this cam 242. Since the moving platform isparallel to the beam, the orientation of the moving platform is alsocontrolled. The same operation can be performed on the other cable 228.When the cam 242 moves towards the edge of the robots, it pulls the beam222 on one side and forces the beam 222 to tilt towards the edge of therobot, which leads to the same rotation on the moving platform 22.

As used herein, the terms “comprises”, “comprising”, “including” and“includes” are to be construed as being inclusive and open ended, andnot exclusive. Specifically, when used in this specification includingclaims, the terms “comprises” and “comprising” and variations thereofmean the specified features, steps or components are included. Theseterms are not to be interpreted to exclude the presence of otherfeatures, steps or components.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiments encompassed within the following claims and theirequivalents.

1. A robotic mechanism, comprising: a support base, an end effector anda biasing member having opposed ends and attached at one of said opposedends to the support base and attached at the other of said opposed endsto the end effector; three cables each connected at a first end thereofto said end effector and said at least three cables having second endsbeing attached to an associated positioning mechanism for moving thesecond ends of said three cables to position said end effector in aselected position in space, a length of each of said three cablesbetween said end effector and said associated positioning mechanismbeing fixed, said biasing member applying force on the end effector withrespect to the support base for maintaining tension in said threecables; and said biasing member is pivotally attached to said endeffector with a first revolute joint and is pivotally connected to saidsupport base with a second revolute joint, the first and second revolutejoints having axis of rotation with are parallel, and wherein saidpositioning mechanism includes three rigid link arms each having firstand second ends with the first end of each support arm being pivotallyattached to said support base in such a way that the rigid link armspivot in planes parallel to each other, and wherein the first ends ofeach cable is attached to the end effector and the second ends of eachcable is attached to the second ends of an associated rigid link arm,and wherein said positioning mechanism includes an actuator attached toeach rigid link arm for pivotally moving each rigid link arm for movingsaid cables thereby moving the end effector.
 2. The robotic mechanismaccording to claim 1 wherein two of said rigid link arms are locatedadjacent to each other on one side of the biasing member and the otherwinch is located on the other side of the biasing member, and whereinthe cable attached to a first of the two adjacent rigid link members isattached to the end effector at a position on the other side of thebiasing member, and wherein the cable attached to a second of the twoadjacent rigid link members is attached to the end effector at aposition on the same side of the biasing members as the second rigidlink member, and wherein the cable attached to the rigid link memberlocated on the other side of the biasing member is attached to the endeffector at a position adjacent to the first revolute joint and alignedwith the axis of rotation of the first revolute joint.
 3. The roboticmechanism according to claim 2 wherein said three cables are eachattached to the end effector using three revolute joints, and whereineach revolute joint has an axis of rotation, the three revolute jointsbeing attached to the end effector so the axis of rotation of each ofthe three revolute joints are parallel, and wherein the axis of rotationof the revolute joint attached to the end effector at the positionadjacent to the first revolute joint has its axis of rotation collinearwith the axis of rotation of the first revolute joint.
 4. The roboticmechanism according to claim 1 wherein said first end of each rigid linkarm is pivotally attached to said support base using a revolute joint.5. The robotic mechanism according to claim 2 wherein the actuatorassociated with each rigid link arm is connected to the revolute jointconnected the rigid link arm to the support base, and including acomputer controller connected to each actuator for controlling movementof said end effector.
 6. The robotic mechanism according to claim 1wherein each of the rigid link arms is moved independently of the otherrigid link arms so that the robotic mechanism has three degrees offreedom.
 7. The robotic mechanism according to claim 1 wherein said endeffector includes a mounting mechanism for receiving a tool to bemounted on the end effector.
 8. The robotic mechanism according to claim1 including a synchronizing mechanism connected to the two rigid linkarms on the same side of the bias member so that the two rigid link armsremain parallel to each other during movement so that the roboticmechanism has two degrees of freedom.